Beating the sub-wavelength limit

Oct 27, 2005

Physicists in Spain and Germany have proposed a technique for sending cold atoms through an array of slits that are much narrower than the de Broglie wavelength of the atoms. The phenomenon, which relies on "surface matter waves", could be used to make atomic circuits (Phys. Rev. Lett.95 170406).

Classical particles can only pass though an aperture if they are smaller than the aperture. Quantum particles like atoms can only pass through an opening if their de Broglie wavelength is smaller than the opening. If the de Broglie wavelength is larger than the aperture, the atoms cannot pass through, even if their physical size is smaller than the aperture. Similarly, light can only pass through a slit if its wavelength is smaller than the width of the slit.

In 1998 researchers showed that light could pass through a metallic film perforated with an array of sub-wavelength holes with the help of surface plasmons -- localised pools of excited electrons around the holes. Now, Esteban Moreno, Antonio Fernandez and Francisco García-Vidal of the University of Madrid, Luis Martin-Moreno in Zaragoza and Ignacio Cirac at the Max Planck Institute for Quantum Optics in Garching have applied similar ideas to cold rubidium atoms.

Moreno and co-workers modelled a thin film that contained an array of slits that were 50 nm wide and separated by 800 nm. Since the de Broglie wavelength of the atoms was around 800 nm, transmission through the slits should have been negligible. However, by carefully tuning the van der Waals interactions between the atoms and the surface, and also the dipolar repulsion created by optical fields in the structure, they showed that 100% of the atoms should be able to pass through the slits with the help of surface matter waves. These waves are the atomic analogue of surface plasmons and may appear when a dielectric surface presents a potential well for the atoms. They are running waves that are confined in the direction perpendicular to the surface but they propagate in the parallel direction.

The group says that the experiment it proposes could soon be possible thanks to recent advances in nanotechnology and the control of cold atoms. "Our work could open a new line of research in the field of atom optics," says García -Vidal. "In photonics, people talk of plasmonic circuits that will be able to carry light at very short length scales. Our finding translates this concept to matter waves and perhaps these ideas could be used to implement atomic circuitry."